At present, most battery management systems (BMS) incorporate passive balancing functions that can regularly adjust all series-connected battery cells to a common SOC value. Passive balancing achieves this by connecting a resistor to each battery cell as needed to dissipate energy and reduce the SOC of the battery cell.
    As an alternative to passive balancing, active balancing uses power conversion to redistribute charges among the individual batteries in the battery pack. This can achieve a higher balancing current, lower heat generation, faster balancing time, higher energy efficiency and longer operating distance.
    This article introduces some common active balancing methods and explains the working principles of these methods.

Battery balance
    Even if the initial matching is good, the batteries in the battery pack will undergo capacity changes over time. For instance, batteries at different physical locations in a battery pack may experience varying temperatures or pressures, thereby affecting their capacity. Furthermore, minor manufacturing differences may magnify over time and cause capacity variations. Understanding capacity differences is crucial for identifying the sources of SOC imbalance.
    The variation of the SOC of the battery cell is mainly determined by the battery capacity and the current entering and leaving the battery. For example, when A 4-Ahr battery receives a current of 1 A within 1 hour, the SOC change is 25%, while a similar 2-Ahr battery will experience a 50% SOC change.
    Maintaining SOC balance requires adjusting the charging/discharging current of each battery according to its capacity. The batteries connected in parallel will automatically perform this operation because the current will flow from the high SOC battery to the low SOC battery. In contrast, the current between series-connected batteries is the same. If there is a capacity difference, an imbalance will occur. This is very important because most battery packs have series battery connections, even though they also include parallel connections.
SOC adjustment is applicable to both passive balancing and active balancing.
    Passive balancing reduces the battery SOC by placing resistive loads (commonly BJT or MOSFET transistors) on each battery. However, active balancing uses the switching mode method to redistribute energy among the batteries in the battery pack.
    Due to the increased complexity and cost during the implementation process, active balancing has traditionally been limited to battery systems with higher power levels and/or large-capacity batteries, such as batteries in power stations, commercial energy storage systems (ESS), home energy storage systems, and battery backup devices. Now there are new solutions with significantly reduced costs and complexity, enabling an increasing number of applications to take advantage of the benefits of active balancing.
    The current of passive balancing is usually limited to 0.25 A, while active balancing can support up to 6 A. A higher balancing current enables faster balancing, thereby supporting larger-capacity battery cells, such as those used in ESS. In addition, a higher balancing current supports systems that operate at fast cycles, in which balancing must be completed quickly.
    Passive balance will only consume energy. Active balancing, on the other hand, redistributes energy, thereby significantly enhancing energy efficiency. Passive balancing is only practical during the charging cycle because operations during discharge will accelerate the energy consumption of the battery pack. On the contrary, active balancing can be implemented during charging or discharging.
    The ability to actively balance during discharge can provide more balancing time and allow charges to transfer from strong batteries to weak batteries, thereby extending the operating time of the battery pack (Figure 2). In conclusion, active balancing is beneficial for applications that require faster balancing, limited heat load, improved energy efficiency, and increased system operating time.

Figure 3 shows that the transformer-based bidirectional active balancer can transfer charges in both directions and can use a 24V power rail. Source: Monolithic Power Systems